Despite different approaches to describing population structure, nearly all research on this topic has assumed that social interactions and behavioral spread are bi-directional [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}.
That is, if Charlie provides a benefit to Bob when behaving as a cooperator, Bob is presumed to provide a benefit to Charlie when acting as a cooperator; moreover, if Charlie has a chance to imitate Bob's behavior, then so too can Bob imitate Charlie's behavior.
The assumption of bi-directionality simplifies analysis and enables simple intuitions
for why population structures permit the evolution of cooperation [5]}, [17]}.
